1. Field of the Invention
The present invention relates generally to Raman lasers, and particularly to pulsed cascaded Raman lasers operating at least in the mid-infrared (IR) wavelength range.
2. Technical Background
Coherent light sources in 1.8-2.0 μm wavelength range and beyond in the mid-IR (2-10 μm) find a number of important applications (such as medicine, life sciences, spectroscopy, and environmental sensing). Important applications also exist for even higher wavelength ranges past the end of the theoretical mid-IR wavelength of 10 μm. However, in contrast to the widely available light sources developed for the visible and near-IR spectral ranges, the choice of the longer wavelength light sources is very limited. InGaAsP/InP based semiconductor lasers, both edge-emitting and vertical-cavity surface emitting (VCSEL) lasers are limited to the operational wavelength shorter than 1.8 μm. InSb/InGaAsSb based and lead salt semiconductor lasers, as well as recently developed quantum cascade lasers, can operate in the mid-IR wavelength range but are limited to a very low output power and/or operation at cryogenic temperatures. Solid state and fiber lasers are limited to the available radiative transitions of the corresponding rare-earth ions (for example, 1.9-2.0 μm for Tm:ZBLAN and 2.9 μm for Er:YAG).
A recently developed technology of Raman wavelength shifting in optical fibers can in principle produce lasing or optical amplification at any wavelength where the glass material used to make the fiber is transparent. Multiple-order stimulated Raman scattering (SRS) generation, where the n-th Stokes component of the initial wavelength serves as a pump for generation of the (n+1)-th Stokes component is a known cascading wavelength converter for accomplishing a significant wavelength shift to a desired region within a transparency window of the glass material used to make the optical fiber, utilizing low cost initial laser sources.
However, the main drawback of known continuous wave (CW) Raman wavelength converters is the requirement to have a pair of high reflectivity mirrors (usually fiber Bragg gratings) defining a high quality optical cavity for each intermediate Stokes component, and associated decrease in a conversion efficiency with the increasing number of SRS cascades (Stokes orders). Practical application of this technology requires producing very high reflectivity (>99%) fiber gratings. Two gratings forming a high quality optical resonant cavity have to be employed for each intermediate conversion wavelength (Stokes order) and conversion efficiency is rapidly decreasing with the increasing number of conversion steps.
Although multiple hundred watt fiber lasers have recently become available, it might be difficult in practice to propagate that much power in a nonlinear fiber (with large enough Raman gain) and avoid stimulated Brillouin scattering in a backward direction.
Therefore, a need still exists to develop high power, efficient and tunable laser sources in the 1.8-10 μm range.